The Shure SM57 and SM58 have the same transducer element, a Unidyne III. The major difference is the grille, which is both cosmetic and functional. Dynamic microphones exhibit a strong "proximity effect", wherein the frequency response is greatly affected by how close the mic's element is to the audio source. The SM57 is commonly used on instruments because the flatter grille allows the mic's transducer to be closer to the audio source than the SM58's ball grille. Also, the spheroid shape of the SM58 grille acts as a "pop" or "breath" filter to help suppress plosives (hard consonants like "P" and "K"). The SM57, with its smaller and tighter grille, focuses audio into the element; most engineers will tell you there is a noticeable increase in the 5KHz - 8KHz range when using that mic versus the SM58.
I like the SM57 for male vocalists who have good mic technique. It's trickier than the more forgiving SM58 as a general vocal mic.
Echoing the other posts above, either microphone will serve you well as a workhorse vocal mic. The SM58 is arguably the most widely used microphone in the world. It's inexpensive (you can find them on Musician's Friend for around $100), rugged, and fairly forgiving. There are much better microphones out there, if you're willing to spend more. As dpeterson points out above, the Shure Beta series is nice, and so are the EV ND mics.